The invention relates to a device for releasing metal vapour of an alkali metal upon heating.
Such devices (dispensers) are used, for example in tubes comprising photocathodes (brightness intensifiers, X-ray image intensifiers) and photomultiplier tubes to deposit a thin layer of the metal, for example cesium on the cathode decrease the work function of the electrons emitted by the cathode. This type of dispenser may also be used in display tubes comprising semiconductor cathodes.
A device of the type mentioned in the opening paragraph is described in GB 1,265,197 in which the powder comprises an alkali chromate such as cesium chromate. When heated the chromate powder is decomposed so that pure cesium is released.
One of the drawbacks of such a device is that the dimensions of the pulverulent grains of the chromate are so small that the powder exhibits poor flow properties, making it difficult to fill the holders in a regular manner and making it difficult to manufacture the dispensers in a reproducible manner.
A second drawback is the emission of unwanted gases during the supply of the alkali metal. Such dispensers often comprise silicon and zirconium-aluminium in addition to the chromate for binding oxygen which is released during the decomposition reaction Zirconium-aluminum in particular emits absorbed hydrogen and hydrocarbon gases at the decomposition temperature of the various alkali chromates (700.degree.-800.degree. C. Also the envelope, which usually consists of nickel-chromium steel, emits these absorbed gases, notably carboncontaining. Particularly the latter gases have a detrimental influence on the operation of photocathodes and semiconductor cathodes.
Moreover, since the alkali metal is supplied by decomposition, the supply of the alkali metal is difficult to control or is not controllable at all.